1. Field of the Invention
The present invention relates generally to a base band circuit of a receiver and a low cut-off frequency control method. More particularly, the invention relates to a direct conversion type receiver having a high-pass filter for blocking transmission of a direct current offset in the base band circuit.
2. Description of the Related Art
A direct conversion type receiver is expected to widely spread toward the future for capability of simplification of a high frequency circuit portion to permit decreasing of number of parts, such as filter and so forth, and capability of execution of most functions, such as band restriction, AGC (automatic gain control) and so forth, within a base band range so that most of the functions can be realized by CMOS analog circuits for facilitating packaging into LSI, as compared with the conventional super-heterodyne type receiver.
FIG. 7 is a block diagram showing a particular construction of the direct conversion type receiver of this kind. In FIG. 7, a high frequency signal received by an antenna 401 is subject to band restriction by a high frequency band-pass filter 402 for extracting a signal of reception band. A band restricted signal is amplified by a low noise amplifier (LNA) 403, and then input to an orthogonal demodulator 404 as it is. The orthogonal demodulator 404 is driven by a local signal generated by a local oscillator 425. The local signal is the same as a center frequency of the high frequency signal to be received. By the orthogonal demodulator 404, the base band signal is directly generated from the high frequency signal. In FIG. 7, the reference numeral 431 denotes a balanced amplifier, 432 and 433 are frequency mixers and 434 is a phase shifter.
The base band signal is two series signals of I and Q. After band restriction of the base band signal by base band filters 405 and 406, the base band signal is amplified by an AGC circuit 407 so that an average amplitude becomes constant. A circuit controlling a gain and algorithm therefor are not directly related to the present invention and detailed discussion for this circuit will be eliminated. A dynamic range of the AGC circuit 407 reaches several tens dB (in case of CDMA (Code Division Multiple Access) system, the dynamic range is about 80 dB). Outputs of the AGC circuit 407 are output to later stages as respective signals 423 and 424. It should be noted that the reference numerals 408 to 411 and 412 to 415 are voltage controlled variable gain amplifiers which are controlled by a control circuit 416. The reference 422 is an external gain control signal.
In the direct conversion system, a channel filter for restricting an adjacent channel is not a SAW filter of an IF band but are realized by base band filters 405 and 406. Since these are realized by a circuit employing an active element, it is adapted for packaging into IC. On the other hand, since the high frequency is directly converted into the base band, a second local oscillator becomes unnecessary. Therefore, all reception circuit from the LNA 403 to a base band output can be integrated as a single chip IC package. This contributes to down-sizing of a cellular telephone and to reducing number of parts.
However, in the filters 405 and 406 and the AGC circuit 407, under presence of a direct current offset even a little, a gain of AGC reaches as large as 80 dB in some case to cause saturation where the output is retained at a power source level or ground level. For example, assuming that a direct current offset of 1 mV is present at the filter 405 and a gain of the AGC circuit 407 is 80 dB, namely ten-thousand times, 10V of direct current component appears on the output of the AGC circuit 407. Of course, in case of the cellular phone or the like, such voltage is far beyond a voltage of a battery, the cellular phone or the like causes failure in operation. Accordingly, in the base band circuit of the direct conversion type receiver, it is the most important task to remove the current offset as much as possible.
The simplest method for removing the direct current offset is C-cut (blocking of direct current by capacitors) as shown in FIG. 8. In the circuit of FIG. 8, high-pass filters 305 to 307 corresponding to C-cut are inserted between VGAs (Variable Gain Amplifiers) forming the AGC circuit or between outputs of VGAs. In FIG. 8, a filter 301 is a low-pass filter performing band restriction of the base band and a transfer function of the filter 301 is assumed as H(s). Since the low-pass filter per se is not directly related to the present invention, detailed discussion thereof is eliminated from the disclosure. In practice, the constructions shown in FIG. 8 are present in both of two series I and Q. Since both are identical circuits, discussion will be given hereinafter for only one series. By the circuit of the shown construction, transfer of the direct current offsets generated at various portions to be output side are prevented. Here,a transfer function of the band bass filter is assumed as B(s).
However, in the C-cut, it is required to insert a plurality of high-pass filters for certainly removing direct current offset component generated in respective portions, as shown in FIG. 8. In order to transfer the signal to later stage with high fidelity, it is desirable to set a cut-off frequency of the high-pass filter as low as possible. With the shown construction, static direct current offset can be prevented substantially completely.
However, the following problems are encountered in practice. For example, assuming that an input converted offset of the variable gain amplifier (hereinafter referred to as VGA) 304 (direct current offset as converted at point “a”) is Vofs, and further assuming that a value of Vofs is not variable in time, a direct current voltage is constant as shown by “a” in FIG. 9. Here, it is assumed that the gain of the VGA 304 is initially one time (0 dB), is varied to be ten times (20 dB) at a time “t0”. A waveform “b” in FIG. 9 shows a voltage at an output point “b” of the VGA. As shown by the waveform “b”, the voltage at the point “b” should be suddenly varied from Vofs to 10×Vfos at the time t0.
A high-pass filter output (voltage at point “c”) produced by C-cutting the waveform by a high-pass filter 307 becomes a waveform shown by a solid line “c” of FIG. 9. As can be clear from this waveform, a static direct current offset voltage can be removed by C-cut. By fluctuation of the input converted offset and gain of the VGA, transitional waveform appears at the output. This waveform also becomes interference in processing of the base band signal in the demodulation circuit in the later stage (which demodulation circuit is not explicitly disclosed in the present invention).
A peak value Vpeak in transition can be expressed by the input converted offset Vofs of the VGA and gains g0 and g1 before and after the VGA as follow:Vpeak=(g1−g0)×Vofs  (1)Namely, the peak value is greater at greater fluctuation of gain.
On the other hand, concerning a continuation period of the waveform “c”, it is assumed that a convergence period of the voltage to 1% of the peak value Vpeak is τ, the high-pass filter 307 is a linear filter and a cut-off frequency is fc, the convergence period τ can be expressed byτ=−ln(0.01)/2 πfc  (2)For example, fc is 10 kHz, τ becomes about 73.3 μsec. This value corresponds to about 281 chips (assuming a chip rate is 3.84 Mcps) in W-CDMA. This period is considerably long. When variation of gain is significant, such long period of τ causes deterioration of a bit rate. In contrast to this, if fc is 1 MHz, τ becomes 0.733 μsec. Thus, convergence period of transition to less than or equal to about 1% of the peak value can be restricted within two to three chips.
However, in the normal state where variation of the gain is small, there is a demand to set the cut-off frequency as low as possible. Namely, control is required in such a manner that    (1) when variation of the gain is sufficiently small (e.g. ≦6 dB), the low cut-off frequency is set as low as possible (e.g. about 10 kHz).    (2) when variation of gain exceeds a predetermined value (e.g. >6 dB, the low cut-off frequency is set higher (e.g. about 1 MHz) for quickly converging the direct current offset in transition (e.g. as waveform shown by broken line d of FIG. 9).
On the other hand, while discussion has been given for a measure in transition due to gain fluctuation of the VGA 304, transition is equally caused in VGA 302 and VGA 303 if fluctuation of the gain is caused and is output via the high-pass filters 305 to 307. Accordingly, similar problem can be encountered to require similar measure.